I've asked this already in Pass Labs, but then thought this might be a more natural home...

I need to drive a piezo actuator (part of an optical filter) with a 30V pk-pk sine wave at between 20 kHz and 50 kHz. The spec sheet says the piezo will show as a capacitance of 0.4 uF. I can envisage one issue with the low-pass filter class-D amps have to filter away the PWM frequencies. Other than that, would a class-D be able to drive a load like this?

It varies with output amplitude but that will also vary with frequency as a fixed voltage will generate different output amplitude at different frequencies but it's pretty linear and the impedance from the capacitance will be the main effect except at the frequencies you're looking into above the audible range.

For example, a piezo like an old Motorola KSN1001 is electrically identical to a 33ohm resistor in series with a 0.3uF capacitor, so at the frequncies you're looking into the resistance part will be the main part of the impedance.

It varies with output amplitude but that will also vary with frequency as a fixed voltage will generate different output amplitude at different frequencies but it's pretty linear and the impedance from the capacitance will be the main effect except at the frequencies you're looking into above the audible range.

For example, a piezo like an old Motorola KSN1001 is electrically identical to a 33ohm resistor in series with a 0.3uF capacitor, so at the frequncies you're looking into the resistance part will be the main part of the impedance.

excellent, thanks Saturnus, that should help my search for an amp, but maybe I'll need to go higher in voltage also